Problem: The scale on a map is 2cm : 4km. If two cities are 12cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 2cm on the map corresponds to an actual distance of 4km. A distance of 12cm on the map is the same as 6 $\cdot$ 2cm. The actual distance between the two cities is 6 $\cdot$ 4km, or 24km.